The present invention relates to a radiation computed tomography apparatus (referred to as a radiation CT apparatus hereinbelow) for calculating a tomographic image of a subject from radiation projection data on the subject.
Known radiation CT apparatuses include an X-ray CT apparatus employing X-rays as radiation, for example. Known X-ray CT apparatuses include an apparatus comprising a plurality of X-ray detectors arranged in a two-dimensional manner. The plurality of X-ray detectors are disposed to have a width in a direction along a predefined axis with respect to a subject. Since rows of X-ray detectors are formed in the axis direction in the predefined width, the X-ray detectors arranged in a two-dimensional manner are generally referred to as a multi-row detector.
One known technique for reconstructing a tomographic image employing the multi-row detector is a cone BP (backprojection) process (see Patent Document 1, for example). In the cone BP process, a scan generally referred to as a helical scan is conducted, in which X-rays passing through a region to be examined in a subject are detected by, for example, rotating the multi-row detector around the aforementioned axis while moving the multi-row detector relative to the subject in the axis direction.
Production of a tomographic image corresponding to a row of the X-ray detectors by the cone BP process uses not only detected data obtained by that row of X-ray detectors but those from X-rays passing through the region to be examined corresponding to the tomographic image and detected by the other rows of X-ray detectors.
[Patent Document 1]
Japanese Patent Application Laid Open No. 2001-161678.
When conducting the helical scan and conducting the cone BP process, if the helical pitch is smaller than a certain value, there is a possibility that some detected data are not used in reconstructing a tomographic image. The helical pitch is a value derived from the relationship between the width of the multi-row detector in a direction along the axis and the distance of movement of the subject relative to the multi-row detector during one rotation of the multi-row detector.
If some detected data are not used in reconstructing a tomographic image, X-rays for acquiring such non-used detected data are uselessly emitted.